The present invention relates to a nuclear reactor and more particularly to a tank type fast breeder reactor improved in its structure and maintenance or inspection performance.
As well known, a conventional tank type fast breeder reactor generally comprises a reactor vessel having an inner sealed space in which a coolant such as liquid metal is circulated and a reactor core (merely called a core hereinlatter) disposed in the reactor vessel. The interior of the reactor vessel is divided by a partition wall structure into a hot pool region disposed on the upper side of the core and a cold pool region disposed below the hot pool region. An intermediate heat exchanger carrying out a heat exchanging operation between the coolant and a secondary heat medium (secondary coolant) is disposed in the hot pool region and a pump means for feeding the coolant to the core is disposed in the cold pool region.
When such fast breeder reactor is operated, the coolant is fed from the cold pool to the core by the operation of the pump means and then heated in a fuel assembly constituting a core. The coolant is first flown upwardly in the core and then drained into the upper hot pool. The coolant drained into the hot pool is subjected to the heat exchanging operation with the secondary heat medium in the intermediate heat exchanger, through which the coolant is then drained into the cold pool.
Such a tank type fast breeder reactor is shown in, for example, "Construction of the world's first fullscale fast breeder reactor" M. Banal (Nersa/EdF) et al., Nuclear Engineering International, May 1978.
In the reactor shown in such literature, the partition wall is built from the inner peripheral surface of the core in an inverted cone shape, and eight intermediate heat exchangers and four pumps are hung from a reactor roof through the partition wall structure in penetrating manners. In the general conventional technology for the tank type reactor, many machinaries and equipments are secured to the partition wall structure in penetrating manner.
In inspections of the tank type reactor, these machinaries and equipments have been visually inspected at the operation stopping state of the reactor, and in such case, when a coolant essentially formed of an opaque liquid metal fills in the reactor vessel, the visual inspection could not be adequately performed. Accordingly, the inspections are carried out by lowering the liquid level of the coolant in the core directly below the core top portion so that the upper portion of the core top portion at which many machineries and equipments are arranged is exposed externally.
In such inspection operation of the described tank type reactor, however, all the fuels arranged in the core had to be preliminarily taken out from the reactor vessel in order to lower the liquid surface level of the coolant below the core top portion, thus being inconvenient and troublesome.
In the case of stopping the reactor operation for the periodical inspection or maintenance, decay heat is generated from the fuel in the core, and hence, it is necessary to cool the coolant for the predetermined period after the stopping of the reactor operation. In this viewpoint, in the conventional tank type reactor, the removal of this decay heat is carried out by the intermediate heat exchangers. However, the intermediate heat exchangers are arranged, because of their function, at an upper portion of the hot pool region, and hence, the intermediate heat exchangers cannot attain their function when the liquid surface level of the coolant is lowered below the core top portion. In this meaning, it is also necessary to take out all the fuels from the reactor vessel for lowering the liquid surface level of the coolant, resulting in the reactor operation stopping for relatively long period, thus also being inconvenient and troublesome.
Furthermore, as described hereinbefore, the conventional tank type reactor has a complicated structure in which many machinaries or equipments such as heat exchangers and pumps are arranged in penetration through the partition wall structure. Such complicated structure requires much labor for the removal, inspection and maintenance thereof in addition to the long operation stopping period of the reactor, thus being inconvenient in the reactor running, economical and other points.